Light flux diameter expanding element and image display device

ABSTRACT

A light flux diameter expanding element includes a light guiding plate with a light input face and a light output face, and with a thickness of 0.2 mm to 0.8 mm; a diffraction grating on the input side; and a diffraction grating on the output side, and is provided so as to have the same grating period as that of the diffraction grating on the input side, in which a forming region of the diffraction grating on the input side is smaller than that of the output side, and a grating period of the diffraction grating on the input side is a period in which a small diffraction angle in diffraction angles of +1-st order diffracted light and −1-st order diffracted light, which are diffracted in the diffraction grating on the input side, in the light guiding plate becomes larger than a critical angle of the light guiding plate.

This is a Continuation of U.S. application Ser. No. 15/433,618 filedFeb. 15, 2017, which in turn claims the benefit of JP 2016-036787 filedFeb. 29, 2016. The disclosure of the prior applications is herebyincorporated by reference herein in its entirety.

BACKGROUND 1. Technical Field

The present disclosure relates to a light flux diameter expandingelement and an image display device.

2. Related Art

In recent years, a mounted-type image display device such as a headmounted display has received attention. As such a head mounted display,a device which draws an image on the retina of the eyes by scanning alaser beam is known (for example, refer to JP-A-2007-264555).

In a head mounted display, image light is expanded by dividing a laserbeam into a plurality of beams using a diffraction grating, it isensured that a laser beam enters the pupils even when the eyes of anobserver are moving a little, and the observer is caused to visuallyrecognize an image.

However, it is not easy to sufficiently expand image light in the abovedescribed related art, since image light is smaller than a diameter of apupil in a method in which a laser beam is used. Therefore, it isdesirable to provide a new technology in which it is possible to causeimage light to be input to a pupil, even in a case in which the imagelight is sufficiently smaller than the diameter of pupil, as in themethod in which the laser beam is used.

SUMMARY

An advantage of some aspects of the embodiment is to provide a lightflux diameter expanding element and an image display device in which itis possible to cause light to be satisfactorily input to a pupil byexpanding the light.

According to an aspect of the embodiment, there is provided a light fluxdiameter expanding element which includes a light guiding plate with alight input face and a light output face, and with a thickness of 0.2 mmto 0.8 mm; a diffraction grating on an input side which is provided onthe light input face; and a diffraction grating on an output side whichis provided on the light output face, and is provided so as to have thesame grating period as that of the diffraction grating on the inputside, in which the diffraction grating on the input side is smaller thanthe diffraction grating on the output side, and a grating period of thediffraction grating on the input side is a period in which a smalldiffraction angle in diffraction angles of +1-st order diffracted lightand −1-st order diffracted light, which are diffracted in thediffraction grating on the input side, in the light guiding platebecomes larger than a critical angle of the light guiding plate.

According to the light flux diameter expanding element in the aspect, itis possible to increase one laser beam input to an input face to aplurality of laser beams, in a state in which an input angle of thelaser beam with respect to the input face is maintained. Since athickness of the light guiding plate is set to 0.2 mm to 0.8 mm, it ispossible to preferably adjust intervals of the plurality of laser beamsto intervals (2 mm or less) smaller than a diameter of the pupils of theeyes.

Accordingly, it is possible to cause at least one light beam to be inputto pupils of eyes, even when the eyes of an observer are moving.

In the light flux diameter expanding element according to the aspect,when a shortest wavelength in wavelength bands of input light which isinput to the light input face is set to λ_(min), an absolute value of amaximum angle of an input angle of the input light with respect to thelight input face is set to |θ_(max)|, and a grating period of the lightinput face is set to P, it is preferable that a grating period of thelight input face satisfy P≤λ_(min)/(sin|θ_(max)|+1).

According to the configuration, it is possible to cause light which isinput to the diffraction grating on the input side to be propagatedinside the light guiding plate using total reflection. In this manner,it is possible to preferably expand a diameter of a light flux which isoutput from the light output face.

In the light flux diameter expanding element according to the aspect, itis preferable that a height of a grating of the diffraction grating onthe output side is lower than a height of a grating of the diffractiongrating on the input side.

According to the configuration, a diffraction efficiency of thediffraction grating on the output side is set to be lower than that ofthe diffraction grating on the input side. Accordingly, it is possibleto cause light which is propagated inside the light guiding plate to beoutput from an output face in a plurality of places, using thediffraction grating on the output side. In this manner, it is possibleto preferably obtain a function of expanding a light flux diameter.

According to another aspect of the embodiment, there is provided animage display device which includes an image light generation unit whichoutputs image light; and an image light expanding element which isconfigured of the light flux diameter expanding element according to theabove described aspect.

According to the image display device in the aspect, since the lightflux diameter expanding element is provided, it is possible topreferably cause an observer to visually recognize image light, even ina case in which pupils of the observer are moving.

In the image display device according to the aspect, it is preferablethat the image light include light with different wavelength bands, aplurality of the image light expanding elements be provided, and gratingperiods of each of the diffraction gratings on input side of theplurality of image light expanding elements be different from eachother.

According to the configuration, it is possible to set a diffractionangle of light in different wavelength bands in the respective lightguiding plates to be the same.

In addition, it is preferable that each of the light guiding plates ofthe plurality of image light expanding elements be provided so as tohave the same thickness as each of the others.

By doing so, it is possible to cause light to be output from the sameposition, regardless of wavelength bands of the light.

In the image display device according to the aspect, it is preferablethat a grating period of the diffraction grating on the input side ofeach of the plurality of image light expanding elements become largerwhile being separated from the image light generation unit.

According to the configuration, it is possible to minimize occurrencesof unnecessary diffracted light.

In the image display device according to the aspect, it is preferablethat the plurality of image light expanding elements include a firstimage light expanding element, a second image light expanding element,and a third image light expanding element, a grating period of thediffraction grating on the input side of each of the first image lightexpanding element, the second image light expanding element, and thethird image light expanding element be a period in which a diffractionangle of diffracted light which is diffracted in each of the diffractiongratings on input side become the same angle, and the image lightinclude light in a first wavelength band corresponding to the firstimage light expanding element, light in a second wavelength bandcorresponding to the second image light expanding element, and light ina third wavelength band corresponding to the third image light expandingelement.

According to the configuration, it is possible to display a color image.

In the image display device according to the aspect, it is preferablethat the image display device further include an ocular optical systemto which light output from the plurality of image light expandingelements is input, and the ocular optical system have light-transmittingproperties.

According to the configuration, it is possible to visually recognizeoutside light (see-through light), along with image light.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiment will be described with reference to the accompanyingdrawings, wherein like numbers reference like elements.

FIG. 1 is a diagram which illustrates a use form of an HMD according toa first embodiment.

FIG. 2 is a perspective view of the HMD according to the firstembodiment.

FIG. 3 is a diagram which illustrates a configuration of an imagedisplay unit.

FIG. 4 is a perspective view which illustrates a configuration of animage light expanding unit.

FIG. 5 is an explanatory diagram in a case in which light with an inputangle 0° is input to an image light expanding element including a lightguiding plate with a thickness of 0.8 mm.

FIG. 6 is an explanatory diagram in a case in which light with an inputangle 0° is input to the image light expanding element including a lightguiding plate with a thickness of 0.2 mm.

FIG. 7 is an explanatory diagram in a case in which light with an inputangle +10° is input to the image light expanding element.

FIG. 8 is an explanatory diagram in a case in which light with an inputangle −10° is input to the image light expanding element.

FIG. 9 is a diagram which illustrates a sectional configuration of animage light expanding unit according to a second embodiment.

FIG. 10 is a diagram which illustrates a sectional configuration of theimage light expanding unit according to the second embodiment.

FIG. 11 is a diagram which illustrates a sectional configuration of theimage light expanding unit according to the second embodiment.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

Hereinafter, embodiments will be described in detail with reference todrawings. There is a case in which characteristic portions of thedrawings used in the descriptions below are illustrated by beingenlarged for convenience, in order to make characteristics be easilyunderstood, and a ratio of dimension of each constituent element, or thelike, is not always the same as the actual dimension.

First Embodiment

An image display device according to the embodiment is an example of ahead mounted display which is used by being mounted on head of a user.

In the following descriptions, the head mounted display is brieflydescribed as an HMD.

FIG. 1 is a diagram which illustrates a state in which a user wears theHMD according to the embodiment.

FIG. 2 is a perspective view of the HMD according to the embodiment.

As illustrated in FIG. 1, an HMD 300 according to the embodiment is adisplay which is used when a user wears the display with a sense ofwearing glasses. The HMD 300 according to the embodiment is asee-through-type (transmission type) HMD. According to the HMD 300 inthe embodiment, a user is able to visually recognize an image generatedby an image display unit, and it is also possible to visually recognizean outside image such as outside scenery of the HMD 300.

As illustrated in FIG. 2, the HMD 300 is provided with a display device100 which is formed in a shape similar to glasses, and a control unit(controller) 200 with a size which a user can hold in hands. The displaydevice 100 and the control unit 200 are communicably connected in awired or wireless manner. According to the embodiment, each of an imagedisplay unit for left eye 110A and an image display unit for right eye110B which configure the display device 100, and the control unit 200are communicably connected in a wired manner through a cable 150, andperform a communication of an image signal or a control signal.

The display device 100 is provided with a main frame (device main body)120, and the image display unit for left eye 110A and the image displayunit for right eye 110B. The control unit 200 is provided with a displayunit 210, and an operation button unit 250. The display unit 210displays, for example, various information, an instruction, or the like,which is provided to a user. The main frame 120 is provided with a pairof temple portions 122A and 122B which is used when a user hooks thereofaround ears. The main frame 120 is a member for supporting the imagedisplay unit for left eye 110A and the image display unit for right eye110B.

The image display unit for right eye 110B and the image display unit forleft eye 110A have the same configuration, and each constituent elementin both of the image display units is bisymmetrically disposed. For thisreason, hereinafter, the image display unit for right eye 110B will bedescribed in detail, as the image display unit 110, simply, anddescriptions of the image display unit for left eye 110A will beomitted.

FIG. 3 is a diagram which illustrates a configuration of the imagedisplay unit 110.

As illustrated in FIG. 3, the image display unit 110 is provided with animage light generation unit 19, an image light expanding unit 20, acondensing optical system 13, and an ocular optical system 14. The imagelight generation unit 19 outputs light including image information. Theimage light expanding unit 20 expands a beam diameter of light which isoutput from a light scanning unit 17 which will be described later.

The image light generation unit 19 is provided with a light source unit15, a collimator lens 28, and the light scanning unit 17. The lightsource unit 15 outputs light generated by a semiconductor laser in theinside. The light source unit 15 is provided with a solid light sourcewhich includes at least one of, for example, a semiconductor laser whichoutputs red light, a semiconductor laser which outputs green light, anda semiconductor laser which outputs blue light.

In a case in which the light source unit 15 is provided with a pluralityof the solid light sources (not illustrated) which include thesemiconductor laser outputting red light, the semiconductor laseroutputting green light, and the semiconductor laser outputting bluelight, each color light output from each semiconductor laser ismodulated according to an image signal, the modulated each color lightis composited, and is output from the light source unit 15 as imagelight.

The collimator lens 28 collimates the light input from the light sourceunit 15.

The light scanning unit 17 scans light which is reflected on a mirror16. The light scanning unit 17 is provided with, for example, a MEMSmirror (not illustrated). The light scanning unit 17 changes a postureof the MEMS mirror according to a modulation operation of the lightsource unit 15, and two-dimensionally scans light. In this manner, thelight scanning unit 17 outputs image light including image information.

In the HMD 300 according to the embodiment, a wide view angle isexecuted by generating image light using the light scanning unit 17which includes the MEMS mirror. The light scanning unit 17 according tothe embodiment adopts, for example, a MEMS mirror of which a diameter is1 mm, and an oscillation angle is a half view angle, and is 10°.

The image light expanding unit 20 is a unit for duplicating a light beamof which a view angle (angle of input light beam) is maintained over awide range so that a light beam for viewing an image definitely enterseyes (pupils), even when eyes of an observer move vertically andhorizontally.

FIG. 4 is a perspective view which illustrates a configuration of theimage light expanding unit 20 according to the embodiment. In thedescriptions, it is assumed that monochromatic light (image light) isoutput from the light source unit 15 of the image light generation unit19.

As illustrated in FIG. 4, the image light expanding unit 20 isconfigured of one image light expanding element 12A. The image lightexpanding element 12A includes a light guiding plate 1, a diffractiongrating on the input side 11, and a diffraction grating on the outputside 12.

The light guiding plate 1 is configured of glass, or a parallel flatplate which is formed of an optical resin, or the like. In theembodiment, the light guiding plate 1 is configured of a glass substratewith a refractive index of 1.52.

The light guiding plate 1 includes a light input face 1 a on one facethereof, and the diffraction grating on the input side 11 is provided onthe light input face 1 a. In addition, the light guiding plate 1includes a light output face 1 b on the other face, and the diffractiongrating on the output side 12 is provided on the light output face 1 b.

In the following descriptions, the descriptions will be appropriatelymade using XYZ coordinates. In this case, an X direction corresponds toone direction of a face parallel to the light input face 1 a and thelight output face 1 b of the light guiding plate 1, a Y directioncorresponds to a direction orthogonal to the X direction in a planeparallel to the light input face 1 a and the light output face 1 b, anda Z direction corresponds to a direction orthogonal to the X directionand the Y direction, and a thickness direction of the light guidingplate 1.

The diffraction grating on the input side 11 and the diffraction gratingon the output side 12 are surface relief-type diffraction gratings, andare set so that grating directions and grating periods are the same inthe diffraction grating on the input side 11 and the diffraction gratingon the output side 12. In the diffraction grating on the input side 11,a plurality of gratings 11 a which extend in the X direction are formedat even intervals in the Y direction. In the diffraction grating on theoutput side 12, a plurality of gratings 12 a which extend in the Xdirection are formed at even intervals in the Y direction.

According to the embodiment, grating periods Px of the diffractiongrating on the input side 11 and the diffraction grating on the outputside 12 in the X direction are the same, and grating periods Py thereofin the Y direction are the same, as well. The grating periods Px and Pymay be the same, or may be different. Specifically, according to theembodiment, the grating period Px and the grating period Py in thediffraction grating on the input side 11 and the diffraction grating onthe output side 12 are set to 0.447 μm, respectively. Hereinafter, therealso is a case in which the grating period Px and the grating period Pyare collectively referred to as a grating period P.

Light input to the image light expanding element 12A is diffracted bythe diffraction grating on the input side 11, and is introduced into thelight guiding plate 1. According to the embodiment, a diffraction angleof the diffraction grating on the input side 11 is set so that light ispropagated inside the light guiding plate 1 using total reflection. Adiffraction angle in the diffraction grating on the input side 11 isdetermined according to a diffraction period of the different grating.

According to the embodiment, the diffraction grating on the input side11 and the diffraction grating on the output side 12 configure a gratingpattern in which protrusion-shaped bodies are two-dimensionally disposedin a plane which is parallel to an XY plane. In FIG. 4, d1 denotes aheight of the protrusion-shaped body of the diffraction grating on theinput side 11 (hereinafter, referred to as height of grating) in the Zdirection, and d2 denotes a height of grating of the diffraction gratingon the output side 12.

Subsequently, a function of expanding pupils in the image lightexpanding element 12A will be described. According to the embodiment,light with a predetermined input angle distribution (range from −10° to+10°) is input to the light input face 1 a of the image light expandingelement 12A using the light scanning unit 17.

FIGS. 5 and 6 are explanatory diagrams in a case in which light with aninput angle of 0° is input to the image light expanding element 12A. Inaddition, FIG. 5 is an explanatory diagram in a case in which athickness of the light guiding plate 1 is 0.8 mm, and FIG. 6 is anexplanatory diagram in a case in which a thickness of the light guidingplate 1 is 0.2 mm.

FIG. 7 is an explanatory diagram in a case in which light with an inputangle of +10° is input to the image light expanding element 12A, andFIG. 8 is an explanatory diagram in a case in which light with an inputangle of −10° is input to the image light expanding element 12A. Inaddition, in FIGS. 5 to 8, it is assumed that light L formed of laserlight of which a wavelength is 0.525 μm is input to the image lightexpanding element 12A.

FIGS. 5 to 8 illustrate a section parallel to a YZ plane, and in whichpropagation of light in the Y direction is explained; however, it isassumed that propagation of light similarly occurs also in the Xdirection. That is, it is assumed that light expanded by the image lightexpanding element 12A is two-dimensionally expanded in the X directionand the Y direction.

According to the embodiment, in diffracted light, +1-st order diffractedlight and −1-st order diffracted light of which diffraction efficiencycan be set to be high are focused. In addition, 0-th order diffractedlight is also generated; however, it is preferable to set 0-th orderdiffraction efficiency to be low, since the 0-th order diffracted lightdoes not contribute to expanding of a light flux diameter.

As illustrated in FIG. 5, light L forms plurality of diffracted light(0-th order diffracted light L₀, +1-st order diffracted light L_(P1),and −1-st order diffracted light L_(M1)) in the inside of the lightguiding plate 1, by being diffracted by the diffraction grating on theinput side 11. Specifically, the 0-th order diffracted light L₀ has adiffraction angle of 0°, and is introduced into the light guiding plate1. The +1-st order diffracted light L_(P1) and the −1-st orderdiffracted light L_(M1) have equal diffraction angles of θ₁ and θ₂ toeach other, and the angle is 50.6°.

Light which is propagated inside the light guiding plate 1, and reachesthe diffraction grating on the output side 12 is diffracted in thediffraction grating on the output side 12, and a part thereof(transmitted diffracted light) is taken to the outside from the lightguiding plate 1.

According to the embodiment, the grating periods P of the diffractiongrating on the input side 11 and the diffraction grating on the outputside 12 are set to be the same. For this reason, an output angle oflight when being output from the diffraction grating on the output side12 is also set to an angle 0° which is the same as the input angle,since the diffraction grating on the input side 11 and the diffractiongrating on the output side 12 have the same diffracting force.

The +1-st order diffracted light L_(P1) and the −1-st order diffractedlight L_(M1) which are reflected in the diffraction grating on theoutput side 12 are reflected at the same angle as the angle input to thediffraction grating on the output side 12, that is, at the angle of50.6° which is the same as the diffraction angle, are propagated insidethe light guiding plate 1, and reach the light input face 1 a.

Here, when the diffraction grating on the input side 11 is disposed at aposition at which the reflected diffracted light using the diffractiongrating on the output side 12 reaches, diffracted light occurs, andlight is output to the outside from the light guiding plate 1, and as aresult, an intensity of light which is propagated inside the lightguiding plate 1 decreases.

According to the embodiment, the diffraction grating on the input side11 is provided at a portion (center portion) of the light input face 1 aof the light guiding plate 1, and the diffraction grating on the outputside 12 is provided on the entire face of the light output face 1 b ofthe light guiding plate 1. That is, the diffraction grating on the inputside 11 and the diffraction grating on the output side 12 are formed inregions which overlap in the thickness direction (Z direction) of thelight guiding plate 1, and a size of a forming region of the diffractiongrating on the output side 12 is larger than that of a forming region ofthe diffraction grating on the input side 11.

According to the embodiment, as described above, since the diffractiongrating on the input side 11 is not located at a position of the lightinput face 1 a at which the reflected diffracted light using thediffraction grating on the output side 12 reaches, it is possible topreferably cause the reflected diffracted light using the diffractiongrating on the output side 12 to be propagated inside the light guidingplate 1, by causing the diffracted light to be totally reflected on thelight input face 1 a.

According to the embodiment, since the diffraction angles (50.6°) of the+1-st order diffracted light L_(P1) and the −1-st order diffracted lightL_(M1) become larger than a critical angle (41.4°) which is determinedby the refractive index (1.52) of the light guiding plate 1, the +1-storder diffracted light L_(P1) and the −1-st order diffracted lightL_(M1) are caused to be propagated inside the light guiding plate 1 inthe Y direction, using total reflection.

The image light expanding element 12A duplicates the light L by causingthe +1-st order diffracted light L_(P1) and the −1-st order diffractedlight L_(M1) which are propagated inside the light guiding plate 1 usingtotal reflection to be output a plurality of times, using thediffraction grating on the output side 12, and obtains an expanded lightflux K which is formed of plurality of output light L2.

Here, when primary diffraction efficiency in transmitting of thediffraction grating on the output side 12 is high, most of intensity oflight at a time of a first input of the +1-st order diffracted lightL_(P1) and the −1-st order diffracted light L_(M1) to the diffractiongrating on the output side 12 is output to the outside of the lightguiding plate 1, and an intensity of light which remains in the lightguiding plate 1 decreases. Due to this, an intensity of light which isoutput to the outside of the light guiding plate 1 at a time of a secondinput and thereafter of the +1-st order diffracted light L_(P1) and the−1-st order diffracted light L_(M1), which are propagated inside thelight guiding plate 1, to the diffraction grating on the output side 12remarkably decreases.

For this reason, diffraction efficiency of the diffraction grating onthe output side 12 is set to be lower than diffraction efficiency of thediffraction grating on the input side 11. That is, the height of gratingd2 of the diffraction grating on the output side 12 is set to be lowerthan the height of grating d1 of the diffraction grating on the inputside 11. In addition, the diffraction efficiency of the diffractiongrating on the output side 12 is set according to the number of times oftaking light to the outside of the light guiding plate 1 (the number ofoutput light L2) from the diffraction grating on the output side 12.

The height of grating d1 of the diffraction grating on the input side 11is set to a height in which primary diffraction efficiency becomes highwith respect to the light L which is vertically input (wavelength 0.525μm). For example, in a case in which a refractive index of thediffraction grating on the input side 11 is 1.65, the height of gratingd1 becomes approximately 0.25 μm, and the height of grating d2 is set tobe lower than 0.25 μm.

As described above, according to the embodiment, duplicating of a lightbeam is performed by causing the +1-st order diffracted light L_(P1) andthe −1-st order diffracted light L_(M1) to be propagated inside thelight guiding plate 1, by providing the diffraction grating on the inputside 11 at a center of the light input face 1 a. In contrast to this, ina case in which the diffraction grating on the input side 11 is providedat an end portion of the light input face 1 a, in order to cause the+1-st order diffracted light L_(P1) or the −1-st order diffracted lightL_(M1) to be output from an end portion of the light guiding plate 1,attenuation of light increases, and an intensity of an output light beamat the end portion of the light guiding plate 1 remarkably decreases,since the number of total reflection increases.

According to the embodiment, since a distance from an input position ofthe light L to the end of the light guiding plate 1 is a half of theentire light guiding plate 1, it is possible to suppress a decrease inintensity of output light at the end portion of the light guiding plate1.

Meanwhile, it is said that a diameter of pupils of eyes of an observeris approximately 2 mm, in general.

For this reason, in order to set so that the observer can visuallyrecognize an image, when eyes of the observer move vertically andhorizontally, it is necessary to set intervals of the plurality ofoutput light L2 which configure the expanded light flux K toapproximately the diameter of pupils (2 mm). That is, the image lightexpanding element 12A according to the embodiment is designed so thatthe intervals of the plurality of output light L2 are set to beapproximately 2 mm.

For a unit for making the interval of the output light L2 which is takento the outside from the image light expanding element 12A small, thefollowing method is taken into consideration.

As a first method, a method of setting the grating period P in thediffraction grating on the input side 11 to be large, that is, a methodof setting a diffraction angle to be small is taken into consideration.However, in this case, propagation of light in the inside of the lightguiding plate 1 using total reflection is not possible, since thediffraction angle becomes smaller than the critical angle, and there isa concern that a function of expanding pupils (function of duplicatingoutput light beam) may not be sufficiently obtained.

As a second method, a method of using a material with a high refractiveindex as the light guiding plate 1 is taken into consideration. However,in this case, since glass with a refractive index higher than generaloptical glass (refractive index of 1.52), which is used in theembodiment is expensive, a manufacturing cost rises.

The inventor paid attention to the fact that intervals of the outputlight L2 are regulated by the grating period P and the thickness of thelight guiding plate 1, and found a range of the thickness of the lightguiding plate 1 in which it is possible to set an interval D of theoutput light L2 to 2 mm or less, in a case of setting to the gratingperiod P in which a diffraction angle which satisfies conditions oftotal reflection in the inside of the light guiding plate 1 is obtained.

Specifically, the inventor found that a thickness A of the light guidingplate 1 is set to the range of 0.2 mm to 0.8 mm.

It is possible to generate the expanded light flux K of which a lightflux diameter is 10 mm in which the plurality of output light beams L2are disposed at intervals of 2 mm, as illustrated in FIG. 5, by takingout the output light L2 three times in the +1-st order direction and the−1-st order direction, respectively, in the image light expandingelement 12A in which the grating period P is set to 0.447 μm, and thethickness A of the light guiding plate 1 is set to 0.8 mm.

In this case, it is possible to preferably cause an observer to visuallyrecognize an image, since at least one beam of the output light L2 isinput to pupils, even when eyes of the observer move in a range of 10 mmin which the expanded light flux K is present.

In addition, it is possible to generate the expanded light flux K ofwhich the light flux diameter is 10 mm, in which the plurality of outputlight beams L2 are disposed at intervals D₁ of 0.5 mm, as illustrated inFIG. 6, by taking out the output light L2 eleven times, respectively, inthe +1-st order direction and the −1-st order direction in the imagelight expanding element 12A in which the grating period P is set to0.447 and the thickness A of the light guiding plate 1 is set to 0.2 mm.

Here, the light L and each of the output light beams L2 include Gaussdistribution G with a diameter of approximately 1 mm. For this reason,when intervals D₁ of the output light beams L2 which are adjacent toeach other becomes 0.5 mm, each of the output light beams L2 enters astate of being overlapped, spatially, as illustrated in FIG. 6.

In this case, it is possible to preferably cause an observer to visuallyrecognize an image, since at least four beams of the output light L2 areinput to pupils, even when eyes of the observer move in a range of 10 mmin which the expanded light flux K is present.

As illustrated in FIG. 6, since each output light L2 spatially overlaps,it is possible to cause light with uniform illuminance distribution tobe input to eyes, even when eyes of the observer move in a range of theexpanded light flux K. Accordingly, it is possible for the observer tovisually recognize an image with a good quality with a little brightnessunevenness.

Subsequently, a case in which the thickness A of the light guiding plate1 is deviated from the range of 0.2 mm to 0.8 mm will be described. Forexample, in a case in which the thickness A of the light guiding plate 1is larger than 0.8 mm, the interval of the output light L2 becomeslarger than 2 mm, and the function of expanding pupils becomesinsufficient.

In a case in which the thickness A of the light guiding plate 1 issmaller than 0.2 mm, the number of times of reflection in the outputlight L2 until being output from the end portion of the light guidingplate 1 increases, and an output intensity of the output light L2 at theend portion of the light guiding plate 1 remarkably decreases.Accordingly, unevenness occurs in the intensity of the output light L2,at a center portion and the end portion of the light guiding plate 1,and there is a concern that luminance unevenness may occur in a visuallyrecognized image when eyes of an observer move. When the thicknessbecomes smaller than 0.2 mm, the light guiding plate 1 is easily broken,and handling thereof becomes difficult.

According to the image light expanding element 12A in the embodiment, itis possible to preferably cause an image to be visually recognized, evenwhen the eyes of the observer move by using the light guiding plate 1with the thickness of 0.2 mm to 0.8 mm.

Subsequently, a case in which the light L is obliquely input to theimage light expanding element 12A (case in which input angle is ±10)will be described while referring to FIGS. 7 and 8. In addition, inFIGS. 7 and 8, a sign of an angle in a counterclockwise direction withrespect to the light input face 1 a or the light output face 1 b is setto a positive sign (+), and an angle in a clockwise direction is set toa negative sign (−).

As illustrated in FIG. 7, in a case in which an input angle of the lightL is 10°, diffraction angles of the +1-st order diffracted light L_(P1)and the −1-st order diffracted light L_(M1) using the diffractiongrating on the input side 11 are different. Specifically, a +1-st orderdiffraction angle θ_(P) becomes +62.4°, and a −1-st order diffractionangle θ_(M) becomes −41.4°. In addition, output angles of the +1-storder diffraction angle θ_(P) and the −1-st order diffraction angleθ_(M) from the light guiding plate 1 are +10° which are the same as theinput angle, that is, the light beams are output in the same direction.

A diffraction angle of the −1-st order diffracted light L_(M1) becomessmall in the light L which is input at an input angle of +10°.Accordingly, when an absolute value of the diffraction angle θ_(M) ofthe −1-st order diffracted light L_(M1) becomes larger than the criticalangle 41.4°, the diffraction angle θ_(P) of the +1-st order diffractedlight L_(P1) essentially becomes larger than the critical angle.

According to the embodiment, since the absolute value of the diffractionangle θ_(M) is equal to the critical angle, the +1-st order diffractedlight L_(P1) and the −1-st order diffracted light L_(M1) can bepropagated inside the light guiding plate 1 using total reflection. Inaddition, according to the embodiment, when the input angle becomeslarger than 10°, the diffraction angle θ_(M) of the −1-st orderdiffracted light L_(M1) becomes smaller than the critical angle, and the−1-st order diffracted light L_(M1) is incapable of being propagatedinside the light guiding plate 1 using total reflection.

As illustrated in FIG. 8, in a case in which the input angle of thelight L is −10°, it enters a state in which a relationship of the inputangle 10° in FIG. 7 is vertically inversed. That is, the +1-st orderdiffraction angle θ_(P) becomes +41.4°, and the −1 diffraction angleθ_(M) becomes −62.4°. In addition, output angles of the +1-st orderdiffraction angle θ_(P) and the −1-st order diffraction angle θ_(M) fromthe light guiding plate 1 are −10° which are the same as the inputangle, that is, the light beams are output in the same direction.

Since the diffraction angle θ_(P) of the +1-st order diffraction lightL_(P1) of which the diffraction angle is small in the light L which isinput at an input angle of −10° is equal to the critical angle, the+1-st order diffracted light L_(P1) and the −1-st order diffracted lightL_(M1) can be propagated inside the light guiding plate 1 using totalreflection.

Here, conditions under which the primary diffracted light can bepropagated inside the light guiding plate 1 using total reflection willbe described by using an expression. First, a wavelength of the light Lis set to λ, an absolute value of the maximum angle of the input angleis set to |θ_(max)|, the refractive index of the light guiding plate 1is set to n, and the grating period P in which a diffraction angle ofthe primary diffracted light in the light guiding plate 1 matches thecritical angle is expressed by the following expression.P=λ/[sin|θ_(max)|+1]  Expression (1)

In a case in which the grating period P is set to be constant, thediffraction angle depends on a wavelength of the light L which is input,and the shorter the wavelength, the smaller the diffraction angle.Accordingly, in a case in which the light L which is input haswavelength distribution, when an absolute value of the diffraction angleof the primary diffracted light with respect to a shortest wavelengthλ_(min) in the wavelength bands of the light L is larger than thecritical angle, it is possible to cause the primary diffracted light tobe propagated inside the light guiding plate 1 using total reflectionwith respect to the light L in the entire wavelength band. The gratingperiod P which satisfies the condition can be expressed, using thefollowing expression. In addition, in the grating period P in thefollowing expression, a large period is set to a target, in a case inwhich periods of the grating periods Px and Py are different.P=λ _(min)/[sin|θ_(max)|+1]  Expression (2)

According to the image light expanding element 12A in the embodiment, itis possible to cause an image to be visually recognized, preferably,even when eyes of an observer move, by using the light guiding plate 1with the thickness of 0.2 mm to 0.8 mm, even in a case in which thelight L is obliquely input to the image light expanding element 12A.

Returning to FIG. 3, the condensing optical system 13 is configured of aplurality of lenses (not illustrated), and has a positive power as awhole. The expanded light flux K which is output from the image lightexpanding element 12A is not one thin beam, and is spatially spread. Forthis reason, when being directly input to the ocular optical system 14,aberration or distortion of image occurs due to the ocular opticalsystem 14. In contrast to this, according to the embodiment, lightoutput from the image light expanding element 12A is condensed by thecondensing optical system 13 which has the positive power as a whole,and an intermediate image GM is formed right before the ocular opticalsystem 14. In this manner, it is possible to reduce an occurrence of theaberration or the distortion of image.

The ocular optical system 14 is configured of a power mirror (concavemirror) which has a positive power in a plane including at least themain frame 120 (refer to FIG. 2) and eyes ME of an observer M, or aholographic optical element. The ocular optical system 14 forms an exitpupil in the vicinity of pupils of the eyes ME of the observer M bycollimating light which forms the intermediate image GM, and transmitspart of outside light. For this reason, the observer M can visuallyrecognize a virtual image G1 in the distance, along with the outsidelight.

As described above, according to the image light expanding element 12Ain the embodiment, it is possible to cause the expanded light flux K, inwhich a plurality of output light beams L2 are disposed with a size ofapproximately 2 mm which is smaller than a size of a pupil, to be inputto the eyes ME of the observer M, as image light. Accordingly, even in acase in which the eyes ME of the observer M are moved, it is possible tocause the eyes ME to visually recognize the image light, preferably. Inaddition, since the primary diffracted light is propagated inside thelight guiding plate 1 in a state of total reflection, and is output fromthe diffraction grating on the output side 12, use efficiency of lightbecomes high.

Accordingly, the HMD 300 which is provided with the image lightexpanding element 12A becomes a display which is excellent in imagevisibility, and has high reliability.

Second Embodiment

Subsequently, a second embodiment will be described. A configuration ofan image light expanding element in the embodiment is different fromthat in the above described embodiment. Specifically, in the abovedescribed embodiment, the case in which monochromatic light is inputfrom the light source unit 15 of the image light generation unit 19 hasbeen described; however, in the embodiment, a case in which light beamsof three colors are input from the light source unit 15 of the imagelight generation unit 19 will be described.

FIGS. 9 to 11 are sectional views which illustrate a configuration of animage light expanding element according to the embodiment. FIGS. 9 to 11illustrate sections which are parallel to a YZ plane, and explainpropagation of light in the Y direction; however, it is assumed thatpropagation of light also occurs in the X direction, similarly. That is,light expanded by the image light expanding element is two-dimensionallyexpanded in the X direction and the Y direction.

FIG. 9 is an explanatory diagram of a diffraction function in an imagelight expanding element for blue color 112B, FIG. 10 is an explanatorydiagram of a diffraction function in an image light expanding elementfor green color 112G, and FIG. 11 is an explanatory diagram of adiffraction function in an image light expanding element for red color112R.

As illustrated in FIGS. 9 to 11, an image light expanding element 112 inthe embodiment includes the image light expanding element for blue color(first image light expanding element) 112B, the image light expandingelement for green color (second image light expanding element) 112G, andthe image light expanding element for red color (third image lightexpanding element) 112R.

Since a basic configuration of the image light expanding element forblue color 112B, the image light expanding element for green color 112G,and the image light expanding element for red color 112R is the same asthat of the image light expanding element 12A in the above describedembodiment, common portions are given the same reference numerals, anddescriptions thereof will be omitted.

According to the embodiment, in the image light expanding element 112,the image light expanding element for blue color 112B, the image lightexpanding element for green color 112G, and the image light expandingelement for red color 112R (hereinafter, these are collectively referredto as expanding elements 112B, 112G, and 112R, simply) are disposed inorder along a proceeding direction of the light L, from an input sidetoward an output side of image light. That is, each of the expandingelements 112B, 112G, and 112R is disposed in order from an expandingelement of which a wavelength of corresponding light is short. Inaddition, the expanding elements 112B, 112G, and 112R are configured ofthe light guiding plate 1 with the same thickness.

According to the embodiment, image light (light L) which is output fromthe light source unit 15 of the image light generation unit 19 includesblue light (light in first wavelength band) LB, green light (light insecond wavelength band) LG, and red light (light in third wavelengthband) LR. These blue light LB, green light LG, and red light LR areinput to the image light expanding element 112 at the same input angle(range of) ±10°.

According to the embodiment, the image light expanding element for bluecolor 112B includes a light guiding plate 1B, a diffraction grating onthe input side 11B and a diffraction grating on the output side 12B, andcorresponds to the blue light LB with a wavelength of 0.460 μm.

The image light expanding element for green color 112G includes a lightguiding plate 1G, a diffraction grating on the input side 11G and adiffraction grating on the output side 12G, and corresponds to the greenlight LG with a wavelength of 0.525 μm.

The image light expanding element for red color 112R includes a lightguiding plate 1R, a diffraction grating on the input side 11R and adiffraction grating on the output side 12R, and corresponds to the redlight LR with a wavelength of 0.610 μm.

According to the embodiment, grating periods PB, PG, and PR of the imagelight expanding element for blue color 112B, the image light expandingelement for green color 112G, and the image light expanding element forred color 112R satisfy the following relationship. That is, the gratingperiods PB, PG, and PR become large while being separated from the imagelight generation unit 19.PB<PG<PR

In addition, a height of grating H11 of the diffraction grating on theinput side 11B, a height of grating H12 of the diffraction grating onthe output side 12B, a height of grating H21 of the diffraction gratingon the input side 11G, a height of grating H22 of the diffractiongrating on the output side 12G, a height of grating H31 of thediffraction grating on the input side 11R, and a height of grating H32of the diffraction grating on the output side 12R satisfy the followingrelationship.H11<H21<H31H12<H11H22<H21H32<H31H12<H11<H22<H21<H32<H31

First, the image light expanding element for blue color 112B will bedescribed.

As illustrated in FIG. 9, the image light expanding element for bluecolor 112B causes both of the +1-st order diffracted light and the −1-storder diffracted light with respect to the blue light LB to bediffracted at an angle larger than a critical angle of the light guidingplate 1B (refractive index 1.52), and a grating period P of adiffraction grating is determined so that diffracted light is propagatedinside the light guiding plate 1B. Specifically, according to theembodiment, a grating periods PB (grating period Px and grating periodPy) in the diffraction grating on the input side 11B and the diffractiongrating on the output side 12B are set to 0.392 μm.

Here, as a comparison example, a case in which the expanding elements112R, 112G, and 112B are disposed in this order (order in which gratingperiod becomes small) from the input side toward the output side ofimage light will be described. In this case, since a grating period ofthe expanding element 112R becomes the largest period, for example, whenthe blue light LB is input, a diffraction angle thereof becomes small, alot of unnecessary components of diffracted light are generated by beinginput to the expanding elements 112G and 112B which are disposed in arear stage, and light with a component which is output at an angledifferent from an input angle increases.

In contrast to this, as described above, according to the embodiment,the expanding elements 112B, 112G, and 112R are disposed in this order(order in which grating period becomes large) from the input side towardthe output side of image light. The green light LG and the red light LRare also input to the image light expanding element for blue color 112B;however, since a wavelength of the green light LG and the red light LRis larger than that of the blue light LB, the green light LG and the redlight LR are diffracted at an angle larger than that of the blue lightLB. Accordingly, since inputting of light with an angle different from apredetermined input angle to the image light expanding element for greencolor 112G or the image light expanding element for red color 112R whichare disposed in the rear stage of the image light expanding element forblue color 112B is suppressed, it is possible to suppress a generationof unnecessary diffracted light.

In addition, the height H11 of the diffraction grating on the input side11B is set to a height in which primary diffraction efficiency is highwith respect to the blue light LB with a wavelength of 0.460 μm, anddiffraction efficiency with respect to the green light LG and the redlight LR is low.

Specifically, the height H11 of the diffraction grating on the inputside 11B becomes 0.57 μm, in a case in which a refractive index of thediffraction grating on the input side 11B is 1.65. Meanwhile, the heightH12 of the diffraction grating on the output side 12B becomes lower than0.57 μm. In this manner, it is possible to optimize distribution oflight intensity which is output from the diffraction grating on theoutput side 12B.

In the image light expanding element for blue color 112B, since gratingperiods of the diffraction grating on the input side 11B and thediffraction grating on the output side 12B are equal, a light beam whichis propagated inside the light guiding plate 1 using total reflection,and reaches the diffraction grating on the output side 12B is output atan angle which is the same as the input angle. Accordingly, similarly tothe image light expanding element 12A in the first embodiment, the imagelight expanding element for blue color 112B generates a plurality ofblue light beams LB2 which are output at the same angle as the inputangle. That is, the image light expanding element for blue colorgenerates an expanded light flux which is obtained by duplicating lightoutput at the same angle as the input angle.

The plurality of blue light beams LB2 are diffracted when being input tothe image light expanding element for green color 112G and the imagelight expanding element for red color 112R, and generates unnecessarydiffracted light. When a diffraction angle of the unnecessary diffractedlight is sufficiently larger than 10°, the unnecessary diffracted lightcan be shielded or absorbed in a rear stage.

Since heights of grating of the image light expanding element for greencolor 112G or the image light expanding element for red color 112R areset so that diffraction efficiency with respect to the green light LG orthe red light LR are high, diffraction efficiency with respect to theblue light LB2 is low, and an occurrence of the above describedunnecessary diffracted light is substantially suppressed.

Subsequently, the image light expanding element for green color 112Gwill be described.

As illustrated in FIG. 10, in the image light expanding element forgreen color 112G, both of the +1-st order diffracted light and the −1-storder diffracted light with respect to the green light LG are diffractedat an angle larger than a critical angle of the light guiding plate 1G(refractive index 1.52), and grating periods of diffraction gratings aredetermined so that diffracted light is propagated inside the lightguiding plate 1G.

Specifically, according to the embodiment, the grating periods PG(grating period Px and grating period Py) in the diffraction grating onthe input side 11G and the diffraction grating on the output side 12Gare set to 0.447 μm.

Since a wavelength of the red light LR which is input to the image lightexpanding element for green color 112G is larger than the green lightLG, the red light LR is diffracted at an angle larger than that of thegreen light LG. For this reason, it is possible to suppress anoccurrence of unnecessary diffracted light in the image light expandingelement for red color 112R which is disposed in the rear stage of theimage light expanding element for green color 112G.

The height H21 of the diffraction grating on the input side 11G is setto a height in which the primary diffraction efficiency is high withrespect to the green light LG with a wavelength of 0.525 μm, anddiffraction efficiency with respect to the blue light LB and the redlight LR is low.

Specifically, the height H21 of the diffraction grating on the inputside 11G is set to 0.60 μm in a case in which a refractive index of thediffraction grating on the input side 11G is 1.65. Meanwhile, the heightd2 of the diffraction grating on the output side 12G becomes lower than0.60 μm. In this manner, it is possible to optimize distribution oflight intensity output from the diffraction grating on the output side12G.

In the image light expanding element for green color 112G, the gratingperiods P of the diffraction grating on the input side 11G and thediffraction grating on the output side 12G are equal. For this reason, alight beam which is propagated inside the light guiding plate 1G usingtotal reflection, and reaches the diffraction grating on the output side12G is output at the same angle as an input angle.

Accordingly, the image light expanding element for green color 112Gduplicates a plurality of green light beams LG2 which are output at thesame angle as the input angle. That is, an expanded light flux which isobtained by duplicating light output at the same angle as the inputangle is generated.

The plurality of green light beams LG2 are diffracted when being inputto the image light expanding element for red color 112R, and generatesunnecessary diffracted light. When a diffraction angle of theunnecessary diffracted light is sufficiently larger than 10°, theunnecessary diffracted light can be shielded or absorbed in the rearstage.

Since a height of grating of the image light expanding element for redcolor 112R is set so that diffraction efficiency with respect to the redlight LR becomes high, diffraction efficiency with respect to the greenlight LG2 is low, and an occurrence of the above described unnecessarydiffracted light is substantially suppressed.

Subsequently, the image light expanding element for red color 112R willbe described.

As illustrated in FIG. 11, in the image light expanding element for redcolor 112R, both of the +1-st order diffracted light and the −1-st orderdiffracted light with respect to the red light LR are diffracted at anangle larger than a critical angle of the light guiding plate 1R(refractive index 1.52), and a grating period of the diffraction gratingis determined so that diffracted light is propagated in the Y directionin the light guiding plate 1R.

Specifically, according to the embodiment, grating periods PR (gratingperiod Px and grating period Py) in the diffraction grating on the inputside 11R and the diffraction grating on the output side 12R are set to0.520 μm.

The height H31 of the diffraction grating on the input side 11R is setto a height in which primary diffraction efficiency with respect to thered light LR with a wavelength of 0.610 μm becomes high, and diffractionefficiency with respect to the blue light LB and the green light LGbecomes low. Specifically, the height H31 of the diffraction grating onthe input side 11R becomes 0.70 μm in a case in which a refractive indexof the diffraction grating on the input side 11R is 1.65. Meanwhile, theheight H32 of the diffraction grating on the output side 12R becomeslower than 0.70 μm. Due to this, it is possible to optimize distributionof light intensity output from the diffraction grating on the outputside 12R.

In the image light expanding element for red color 112R, since thegrating periods of the diffraction grating on the input side 11R and thediffraction grating on the output side 12R are equal, a light beam whichis propagated inside the light guiding plate 1R using total reflection,and reaches the diffraction grating on the output side 12R is output atthe same angle as the input angle.

Accordingly, the image light expanding element for red color 112Rduplicates a plurality of red light beams LR2 which are output at thesame angle as the input angle.

According to the embodiment, it is possible to set diffraction angles ineach of the expanding elements 112B, 112G, and 112R to be the same, bysetting grating periods of the expanding elements 112B, 112G, and 112Rto be different in each color, as described above.

According to the embodiment, since thicknesses of the expanding elements112B, 112G, and 112R (light guiding plates 1B, 1G, and 1R) are the same,output positions of output light of each color (blue light LB2, greenlight LG2, and red light LR2) become the same as each other.

Specifically, in a case in which the thicknesses of the light guidingplates 1B, 1G, and 1R which configure the expanding elements 112B, 112G,and 112R are set to 0.8 mm, an expanded light flux K1 with a light fluxdiameter of 10 mm is generated, when transmitting the expanding elements112B, 112G, and 112R. In the expanded light flux K1, intervals of outputlight of each color (blue light LB2, green light LG2, and red light LR2)are set to approximately 2 mm, similarly to that in the firstembodiment.

Here, as a comparison example of the configuration of the embodiment, aconfiguration in which grating periods of the expanding elements 112B,112G, and 112R are set to be the same, and output positions of eachcolor light are set to be the same, by setting the thicknesses of thelight guiding plates 1B, 1G, and 1R to be different is also taken intoconsideration. Specifically, the thicknesses of the light guiding plates1B, 1G, and 1R of which diffraction angles are small are set to belarge.

However, in this case, since the grating period is the same in eachcolor light, diffraction efficiency also becomes different, since thediffraction angle is different in each color light. In particular,diffraction efficiency of the red light LR of which a diffraction anglebecomes large decreases, and it is not easy to adjust a color balance ofeach color light.

In contrast to this, according to the configuration of the embodiment,since the grating period is determined so that a diffraction angle isset to be the same in each color light, diffraction efficiency of eachcolor light also becomes the same, respectively, it is possible toobtain an effect that it is easy to keep a color balance.

Since it is possible to make the diffraction grating using the sameprocess, because the thicknesses of the light guiding plates 1B, 1G, and1R are the same, it is possible to obtain an effect of suppressing amanufacturing cost of each of the expanding elements 112B, 112G, and112R.

As described above, according to the image light expanding element 112in the embodiment, it is possible to cause image light (expanded lightflux K1) in which output light beams of each color (blue light LB2,green light LG2, and red light LR2) are disposed at intervals ofapproximately 2 mm which is smaller than a size of pupil to be input tothe eye ME of an observer M. Accordingly, it is possible to cause theeye ME to visually recognize image light of three colors, even in a casein which the eye ME of the observer M is moved. In addition, sinceoutput positions of each color light are the same, turbulence in colorbalance of image light which is visually recognize by the observer M issuppressed, even in a case in which the eye ME of the observer M ismoved.

Therefore, according to the HMD in the embodiment which is provided withthe image light expanding element 112, it is possible to provide animage display device which is excellent in visibility of a color image,and has high reliability.

A technical range of the embodiment is not limited to the abovedescribed embodiments, and it is possible to add various changes in arange not departing from the scope of the embodiment.

In the above described embodiments, the case in which an input angle ofthe light L is set to ±10° is exemplified; however, a range of the inputangle of the light L is not limited to ±10°.

For example, the light L may be input as an input angle with an angle of±10° or more. In this case, the grating period P, and the thickness ofthe light guiding plate 1 in a range of 0.2 mm to 0.8 mm may beappropriately set so that conditions for total reflection in the lightguiding plate 1 are satisfied.

The entire disclosure of Japanese Patent Application No. 2016-036787,filed Feb. 29, 2016 is expressly incorporated by reference herein.

What is claimed is:
 1. A light flux diameter expanding elementcomprising: a first light guiding plate with a thickness of 0.2 mm to0.8 mm; a first input diffraction grating provided on the first lightguiding plate; and a first output diffraction grating provided on thefirst light guiding plate, and provided so as to have an identicalgrating period as that of the first input diffraction grating, wherein:the first input diffraction grating is smaller than the first outputdiffraction grating, the first input diffraction grating diffracts alight into a first diffracted light and a second diffracted light, thefirst diffracted light is propagated toward a first direction, thesecond diffracted light is propagated toward a second direction that isopposite to the first direction, the first output diffraction gratingdiffracts both the first diffracted light and the second diffractedlight, and a grating period of the first input diffraction grating is aperiod in which a small diffraction angle in diffraction angles of thefirst diffracted light and the second diffracted light, in the firstlight guiding plate becomes larger than a critical angle of the firstlight guiding plate.
 2. The light flux diameter expanding elementaccording to claim 1, wherein a total thickness of the first lightguiding plate and the first input diffraction grating is equal to orlower than 0.825 mm.
 3. The light flux diameter expanding elementaccording to claim 1, wherein a total thickness of the first lightguiding plate, the first input diffraction grating and the first outputdiffraction grating is lower than 0.850 mm.
 4. The light flux diameterexpanding element according to claim 1, wherein a diffraction efficiencyof the first output diffraction grating is lower than a diffractionefficiency of the first input diffraction grating.
 5. The light fluxdiameter expanding element according to claim 1, wherein a height of agrating of the first output diffraction grating is lower than a heightof a grating of the first input diffraction grating.
 6. The light fluxdiameter expanding element according to claim 1, wherein a distance fromthe first input diffraction grating to an end of the first light guidingplate is a half of an entire of the first light guiding plate.
 7. Thelight flux diameter expanding element according to claim 1, wherein adistance from the first input diffraction grating to an end of the firstoutput diffraction grating is a half of an entire of the first outputdiffraction grating.
 8. The light flux diameter expanding elementaccording to claim 1, wherein an interval of an output light whichdiffracted by the first output diffraction grating is 2 mm or less. 9.The light flux diameter expanding element according to claim 1, furthercomprising: a second light guiding plate with a thickness of 0.2 mm to0.8 mm provided over the first light guiding plate; a second inputdiffraction grating provided on the second light guiding plate; and asecond output diffraction grating provided on the second light guidingplate, and provided so as to have an identical grating period as that ofthe second input diffraction grating, wherein: the second inputdiffraction grating diffracts a light which passed through the firstinput diffraction grating, and a grating period of the second inputdiffraction grating is wider than a grating period of the first inputdiffraction grating.
 10. The light flux diameter expanding elementaccording to claim 9, further comprising: a third light guiding platewith a thickness of 0.2 mm to 0.8 mm provided over the second lightguiding plate; a third input diffraction grating provided on the thirdlight guiding plate; and a third output diffraction grating provided onthe third light guiding plate, and provided so as to have an identicalgrating period as that of the third input diffraction grating, wherein:the third input diffraction grating diffracts a light which passedthrough the second input diffraction grating, and a grating period ofthe third input diffraction grating is wider than a grating period ofthe second input diffraction grating.
 11. An image display devicecomprising: an image light generation unit which outputs an image light;and an image light expanding element which is configured of the lightflux diameter expanding element according to claim
 1. 12. The imagedisplay device according to claim 11, wherein the image light includeslight with different wavelength bands.
 13. A light flux diameterexpanding element comprising: a first light guiding plate with athickness of 0.2 mm to 0.8 mm; a first input diffraction gratingprovided on the first light guiding plate; and a first outputdiffraction grating provided on the first light guiding plate, andprovided so as to have an identical grating period as that of the firstinput diffraction grating, wherein: the first input diffraction gratingis smaller than the first output diffraction grating, the first inputdiffraction grating diffracts a light into a first diffracted light anda second diffracted light, the first diffracted light is propagatedtoward a first direction, the second diffracted light is propagatedtoward a second direction that is opposite to the first direction, thefirst output diffraction grating diffracts both the first diffractedlight and the second diffracted light, a diffraction angle of the firstdiffracted light is smaller than a diffraction angle of the seconddiffracted light, and the diffraction angle of the first diffractedlight is larger than a critical angle of the first light guiding plate.